1. Field of the Invention
The present invention relates to rotating electrical machines having auxiliary windings in the stator of the machines for providing multiplanar rotor balancing, mechanical stiffening and rotor levitation/centering independent of machine orientation or axis of rotation.
2. Related Art
Electrical machines, such as motors and generators, may use auxiliary or control windings for rotor levitation/centering and rotor balancing. A typical rotor can have a slight amount of mass imbalance. As the rotor rotates, the mass imbalance leads to an unwanted vibrational force appearing at the motors bearings. Such vibrational forces reduce the lifetime of bearings and degrade the performance of the machine. The electromagnetics associated with the machine can also cause undesired forces which appear at the motor casing due to deflection of the stator core. Prior attempts have been made to cancel the forces created by a mass imbalance and/or electromagnetics by forming a magnetic field with auxiliary control windings in a manner which creates a force on the rotor to cancel the undesired forces.
In conventional electrical machines which incorporate auxiliary windings, the control windings are provided along the entire active length of the machine which provides a uniform force distribution. A single sensor measures the effect of undesired forces at a single point and feeds the information to a controller. The controller then drives the control windings with a controlled voltage or current determined from the measurements made by the sensor. With this winding arrangement, optimizing the performance at one axial location along the rotor based on sensor measurements may make the performance at other locations worse.
Some prior art machines also include radial magnetic bearings. Magnetic bearings cause the rotor to levitate/center and eliminate the need for mechanical contacts. Conventional magnetic bearings are placed only on the spindle, which has a relatively smaller diameter. Such placement necessitates the use of a high magnetic field per unit area or use of axially longer bearing surface (since the magnetic field may be limited by material characteristics) in order to levitate/center the rotor.